1. Field
This disclosure relates generally to cellular structures and, in particular, cellular structures having non-planar shapes. Still more particularly, the present disclosure relates to a method and apparatus for forming a cellular structure using ribbons configured to be folded and joined such that the cellular structure has a tree-dimensional (3D) shape.
2. Background
A honeycomb structure, also referred to herein as a “honeycomb core,” a “core material” or simply a “core,” typically comprises a plurality of abutting rectangular or hexagonal cells shaped to a desired form. Honeycomb structures are often used as structural support for their high strength to weight ratio due to the low density of the honeycomb formation.
Honeycomb structures are typically manufactured from a thin, flat base material such as metal or paper. The flat base material is cut into narrow, elongated strips or ribbons, which are folded or bent into contoured strips of semi-hexagonal peaks and troughs. For example, an elongated strip of a material may be scored at regularly spaced intervals. To form regular hexagonally shaped cells, the score lines would be parallel to the ends of the strip and the material would be folded along the score lines to an angle of about 60°, twice in one direction, then twice in the opposite direction, and continuously alternating in that fashion.
The resulting folded strips are then joined together using adhesive, spot welding techniques, brazing techniques, and/or other known joining methods to form a structure having a series of hexagonally shaped cells, thereby forming a flat or substantially planar honeycomb structure. Although cells in a honeycomb structure are typically hexagonal, honeycomb structures may also be formed from cells having non-hexagonal shapes.
The resulting honeycomb structure, which consists of a substantially planar structure having cells with walls oriented in a direction perpendicular to the flat surface of the structure, may be able to sustain large loads in a direction parallel to the walls of the honeycomb cells, while also being lightweight due to an absence of material within the cells.
In many applications, it may be desirable to form a honeycomb structure that is non-planar. Various methodologies and apparatuses have been developed for shaping honeycomb structures into particular non-planar shapes.
For example, without limitation, some currently available methods for forming curved honeycomb structures begin with a pre-formed flat honeycomb structure and then mold or form this flat honeycomb structure into a desired shape that is non-planar.
As one illustrative example, one method of producing a contour consisting of short angle bends in a honeycomb structure consists of first manufacturing a flat honeycomb core material. A force is applied to cells of the flat honeycomb structure to deform or collapse the honeycomb cells in the area in which the short angle bend is desired. This deformation of the honeycomb cells results in a honeycomb structure having a short radius bend area possessing cells with a height similar to the height of cells in the non-collapsed area.
Other methods of contouring core material consist of passing a pre-formed, flat honeycomb core material through a series of rollers that deforms the hexagonal cells and allows them to be bent in different directions. Still further methods of forming core material into a desired shape consist of beginning with a flat core material and forcing the core material against and into a die having the required contour.
All of the foregoing methodologies require the application of force to a flat honeycomb structure in order to form it into a desired shape, which may lead to undesirable stresses in the honeycomb structure. Further, the strength and stiffness of the core are sacrificed due to the fact that the honeycomb cell walls are no longer normal to the surface of the core.
Other methods generally avoid bending or folding a fully assembled honeycomb core material. Instead, these methods begin by forming flat, rectangular strips having a plurality of sections along the length of the strips, the sections being separated by fold lines. The strips are folded at the fold lines and joined together to form a desired honeycomb contour shape without additional application of force to the honeycomb core.
For example, some methods contemplate the formation of a honeycomb structure having hexagonally shaped cells wherein some cell walls possess a tapering V-shaped crimp. By placing all crimped edges on one side of the honeycomb structure, and all non-crimped edges on the opposite side of the honeycomb structure, the crimped side is made to be shorter than the non-crimped side. This facilitates variation in the radii of curvature of the honeycomb structure, which leads to a curved core material.
Other methods contemplate forming rectangular strips wherein the fold lines are placed along the length of the strips, such that the sections between the fold lines are not regularly shaped. Fold lines are placed in the strips such that when folded, the entire edges of the strips form an overall curved structure. When the folded strips are adhered together, the resulting core material has a desired contour. For example, Japanese Laid-Open Patent Publication No. 58-25531 and U.S. Pat. No. 5,270,095 disclose strips having some fold lines perpendicular to the length of the strip and other fold lines that are slanted in relation to the length of the strip. In a flat or unfolded state, the edges of the strip are straight and form a rectangle. In a folded state, the slanted fold lines create a folded strip with straight edges that form an overall curved structure determined by the angle of the slant in the fold lines. However, this process has limited utilization in that it can be used to manufacture honeycomb core having only a single shape.
What is needed is a simplified method of manufacturing contoured honeycomb structures that does not introduce undesired stresses or sacrifice strength and stiffness of the structure, and permits formation of contoured honeycomb core in a wide variety of shapes and sizes with minimal forming steps to provide manufacturing cost and time efficiencies.